The goals of this research include expediting pharmacological studies toward potential benefits from substance P or an analog to treat disease. Substance P is believed to be a hormone, which was isolated from the tissues of brain and the intestinal tract. It is a peptide having 11 amino acids and the sequence, Arg-Pro-Lys-Pro-Gln-Gln-Phe-Phe-Gly-Leu-Met-NH2. A new modification of the structure of SP is being synthesized toward the achievement of a new and better radioimmunoassay. Many new analogs of SP have been and still are being synthesized. The molecule may be reduced from 11 to 7 amino acids with retention of essentially full biological activity as long as certain sequential requirements are met. An analog of activity greater than that of SP has been recognized, and critical amino acids in the sequence have been determined. Certain modifications of the sequence have provided analogs which have been assayed for antagonistic activity.